Problems with Headspace GC

[Geof235]

Hi,

I am running on an Agilent 6890 GC with a 7694 Agilent headspace auto sampler. The system is old, however, up until recently has been working smoothly. All of a sudden I don't get peaks anymore. It's definitely not the GC itself since I've made manual direct injections and I get peaks. So everything is pointing to the headspace. I checked that flow is coming out of the headspace autosampler so it's not a flow rate problem. I'm thinking sample loop or injector?

My sample is Sodium Starch Glycolate. Here are the parameters

Oven: 80C
Loop: 90C
Transfer Line: 100C
Vial Eq time: 45min
Vial Pressurization time: 0.2min
Injection time: 0.2min
Loop fill time: 0.2min
Loop Eq. Time: 0.2min
GC Cycle time: 22min
Shake: High

Sample is 100mg of SSG in 20mL headspace vial. Add 5.0mL of DMSO.

Please note that this has happened about a week ago. I thought I clogged the headspace autosampler and I cranked the oven, loop, and transfer line up to 150, 160, 170 each to try to get whatever out. It was giving me nice peaks but then all of a sudden no more peaks.

Thanks

Jeff

[rb6banjo]

I'm assuming that this sampler has fixed-volume loops on a rotary valve for the sample introduction. You should be able to remove the loop and blow gas or water through it (for gas, stick the outlet end into a beaker of water and see the gas go through it) to ensure it's not plugged. That will discount the loop as the source of your problem. I doubt it's this but you should make sure. I've been burned by ignoring the obvious before.

On the Agilent website under the information for this instrument there is a "leak check procedure". You should try that.

https://www.agilent.com/en/products/gas ... ce-sampler

My bet is that your transfer line has broken. If you are using helium carrier gas and you have a leak detector, if I'm you I try to snoop all around that transfer line and the fittings to see if I can find a leak. I've never torn one of these apart before but I'm betting that there's something in the manual for the instrument that shows you how to replace it.

[Geof235]

Some other observations. My carrier pressure is 46psi. I have a 5:1 split and my GC flow rate is 5mL/min making my split flow 25mL/min. The total flow on the GC is reading 33mL/min (assuming this is 25mL/min + carrier flow from the headspace).

Should I increase my carrier pressure? Maybe my sample isn't transferring completely over in the headspace?

[ATAlR]

Assuming you are using SSL inlet check the total flow without transfer line an with transfer line the difference should be at least 10% higher with transfer line.

If you have positive change in pressure with transfer line I would assume vent valve is broken. In my experience with few headspaces it is the part that is most prone to failures.

[Geof235]

So I turned up my carrier pressure to 68psi (as high as it will go before it says it can't give an accurate reading anymore) and I'm getting my peaks.

I was always under the impression that as long as your flow is equal to or higher than your GC flow, you wouldn't have an Issue with peaks not carrying over.

Furthermore, I was once told that the split on headspace is controlled by the carrier flow, not by the GC split flow program. I believe this is false information, and that the GC will still split correctly using the split flow program regardless of the carrier pressure?

[Henrylozano]

So I turned up my carrier pressure to 68psi (as high as it will go before it says it can't give an accurate reading anymore) and I'm getting my peaks.

I was always under the impression that as long as your flow is equal to or higher than your GC flow, you wouldn't have an Issue with peaks not carrying over.

Furthermore, I was once told that the split on headspace is controlled by the carrier flow, not by the GC split flow program. I believe this is false information, and that the GC will still split correctly using the split flow program regardless of the carrier pressure?

Hi, test it for leaks and pressure. What may be happening is that the vent valve is damaged.

[HawkEye_Analytical_1]

There is a lot of bad information here. Let's start somewhere important:

How is the transfer line configured? There are two configurations installed by Agilent. One is the transfer line needle and strain relief nut, so that the transfer line of the HS goes through the septa into the GC inlet. The other is a direct plumbing of the HS transfer line into the inlet weldment. This is a stainless steel connection. (not recommended. ask me for details)

Secondly... suddenly stopped? Cranked up carrier pressure???

Some things do not jive here. Assuming you have the needle into septa configuration:

Check the sample probe. The is the needle point that punctures the vial. The hole for gas intake is on the side of the needle, and can get clogged easily, especially if using "Shake" set to "high". Droplets of liquid, especially high boiling solvents, will stick to the septa of the vial, and then coat the needle. Evaporation leaves solids behind and can clog the small needle hole.
Next check you transfer line needle. If "cranking the pressure" means increasing the supply pressure to the Headspace, then you may have a partial clog of the needle. Often bits of septa get stuck inside the transfer line needle. This is usually caused by puncturing the septa, with the strain relief nut tightened too much. This hardens the rubber, and the needle cores pieces away.

Your GC and headspace should have the same supply pressure. Usually about 80 psi. The transfer line flow rate, for the needle configuration, should be 50% of the column flow rate. This can be adjusted in special cases, but it is easy to cause major issues outside of this range.

Recommendations:
Use low shake. High will clog probes.
Your temperature changes are not dramatic enough to warrant a equil time. change loop equil time to zero.
Change inject time and loop fill time to 0.1 min. Increasing these times does not increase peak size. Increasing vial fill time increases risk of carryover. Increasing loop fill time, increases sample vented into the room.
Check the vial septa type. Often folks use "butyl rubber". This is not recommend for the temps you are using. Anything above 100 C should use silicon rubber.
Since you are using DMSO, the loop, transfer line, inlet, column max temp, and detector should all have a temp higher than the boiling point of DMSO.

As far as your last comments:

The GC will split correctly as long as you have glass wool in your liner to mix the sample flow with carrier, AND the transfer line flow is less than the total flow. Keep in mind there is a septa purge flow, so your transfer line should be less than the total flow minus the septa purge. Hence the 50% recommendation. In splitless mode, the peaks will split, if your transfer line flow is greater than the column flow. This is also an issue, if you have EPC for flow control on the GC, the splitless flow when the purge valve is off, travels to the EPC controller without passing through a chemical trap. (expensive repairs coming)

Since you are using DMSO, the loop, transfer line, inlet, column max temp, and detector should all have a temp higher than the boiling point of DMSO.

If you have a direct plumbed system, I'll type a another message.

[Geof235]

Hi,

Months ago, I posted regarding losing peaks on my headspace autosampler and did a number of trouble shooting to determine the reason behind why my GC would all of a sudden not produce any peaks.

I have determined that the source is coming from the sample vent valve on the Agilent 7694 headspace autosampler. It appears that during the injection, the valve will get stuck in the closed position. After reviewing the user manual
https://www.ssllc.com/images/uploads/Ag ... Manual.pdf
under advanced functions, when I try to turn the sample vent valve on and off, the circular dial does not move. I've contacted Agilent for help, but they seemed pretty uninterested in actually helping (probably because this model is set for discontinuation) but confirmed that this is most likely the problem. There is a motor behind the sample valve wheel, but I can't find the part numbers to fix this. Can anyone help? We were able to open the headspace unit up and with a gently touch, we can get the wheel to move, but it will randomly stop throughout the run. We also tried to apply some WD-40 (reluctantly) on the wheel to grease it up, but it didn't appear to help. I think the motor that drives the valve is on it's way out.

Any help on tutorials on how to replace this myself and where I can get replacement parts would be helpful!

Thanks

Jeff

[pwm]

Hi Jeff,

Do you do any maintenance on the rotor seal? Ive seen they can be over tight and cause excessive force on the motor. Also interconnect could be loose, but in that case motor would work, not rotating rotor. I can see if parts are still around unless you have found it by now?

Patrick

[pwm]

Also you may be confusing vent valve with sample valve, it seems you have combined the two into sample vent valve. Sample valve drives the 6 port rotor and turns in a circular motion left and right. Vent valve is a solenoid that opens and closely with a slight audible click, no visible motion on the vent valve.

[pwm]

This may be what youre looking for

425-019-HSP

Still available through Agilent. Id still recommend checking out the load on the motor is correct, just in case the motor itself the not problem. ie could be as simple as over tight or gunked up rotor seal.